NYSOS (or Nysus) was an old rustic rustic god or Seilen who made his home on the mythical Mount Nysa. There he was entrusted with the care of the young god Dionysos, whom he reared with the help of his five daughters, the Nysiades.

Nysos's mountain home was traditionally identified with Mount Kithairon (Cithaeron) in Boiotia. Its southern vales, known as the Nysaian fields, were the site of the abduction of Persephone in the Homeric Hymns. The god Nysos was probably the same as Seilenos, who was the usual nurse of Dionysos, and he was probably also identified with the mountain-god Kithairon.

Pseudo-Hyginus, Fabulae 179 (trans. Grant) (Roman mythographer C2nd A.D.) :
"Jove [Zeus], coming with lightning and thunder, burned Semele to death. From her womb Liber [Dionysos] was born. Mercurius [Hermes] snatched him from the fire and gave him to Nysus [Seilenos] to be reared. In Greek he is called Dionysus."

Pseudo-Hyginus, Fabulae 131 :
"When Liber [Dionysos] was leading his army into India, he gave the authority over his Theban kingdom to his nurse Nysus until he should come back. But after Liber returned from there, Nysus was unwilling to yield the kingdom. Since Liber [Dionysos] didn't want to quarrel with his nurse he permitted him to keep the kingdom until an opportunity should come to recover it. And so, three years later, he made up the quarrel with him, and pretended he wanted to celebrate in the state the sacred rites termed Trieteric, because he performed them after the third year. He introduced soldiers as Bacchanals in women's dress, captured Nysus, and recovered his kingdom."